This invention relates to partially cross-linkable compositions useful in the preparation of thermoplatic elastomers essentially consisting of a monoolefin copolymer rubber and a polyolefin resin and having its rubber component cross-linked partially, and to an improved process for preparing such partially cross-linked thermoplastic elastomers.
Thermoplastic elastomers, which have excellent rubber elasticity and can be made into articles of any desired shape by common molding techniques for thermoplastic resins (such as extrusion, injection molding, etc.), have been attracting attention in the fields of automobile parts, electrical appliance parts, footwear, wire coatings, miscellaneous goods and the like, as rubbers or rubber-like materials that requires no vulcanization process.
While thermoplastic elastomers of the polystyrene, polyester or polyurethane type are prepared by polymerizing a hard segment and a soft segment, thermoplastic elastomers of the polyolefin type are prepared by melt-kneading a polyolefin resin (e.g., polypropylene) as a hard segment and a monoolefin copolymer rubber as a soft segment and, as the case may be, effecting partial cross-linking of the rubber during the melt-kneading. These thermoplastic elastomers have heretofore been developed with respect to the ethylene-propylene rubber/polypropylene resin system and are being positively put to practical use in the manufacture of automobile parts and the like, because they are characterized by relatively low costs and excellent properties such as flexibility, resistance to heat distortion, and weather resistance.
Among others, partially cross-linked thermoplastic elastomers of the polyolefin type have so excellent in resistance to heat distortion, rubber elasticity and the like that they are expected to be the most promising materials of all such thermoplastic elastomers. In order to prepare partially crosslinked thermoplastic elastomers, it is common practice to melt-mix a monoolefin copolymer rubber [e.g., an ethylene-propylene-diene copolymer rubber (EPDM) or an ethylene-propylene copolymer rubber (EPM)] and polypropylene mechanically in a Banbury mixer, kneader or the like and add a cross-linking agent thereto during this melt-mixing.
A number of processes for preparing such thermoplastic elastomers are well known and disclosed, for example, in Japanese Laid-Open Patent Publication Nos. 26838/'73, 145553/'76, 37953/'77 and 145857/'78. In these processes, however, little consideration is given to the method of addition of a cross-linking agent. The only method employed therein is to add a cross-linking agent directly. This direct addition method is poor in workability and, especially in the case of preparation with an extruder, fails to achieve acceptable efficiency in adding the cross-link agent. Among others, cross-linking agents in powder form cannot be easily mixed with granular monoolefin copolymer rubbers or crystalline polyolefin resins and, moreover, cannot provide satisfactory accuracy of the amount added, so that it is hard to obtain products having consistent qualities. Furthermore, in the case of preparation with an extruder, Banbury mixer or the like, the cross-linking agent added thereto may stick to the equipment heated to a temperature in the vicinity of 200.degree. C. or scattered about before it is fully dispersed in the monoolefin copolymer rubber and the crystalline polypropylene resin. Thus, there is a possibility of the cross-linking agent (or organic peroxide) existing locally at very high concentrations. With the cross-linking agent in such a localized state, the composition tends to explode or ignite when it is subjected to shocks or contaminated with foreign substances (e.g., reducing agents such as amines) that accelerates decomposition of the cross-linking agent. In fact, such accidents have been reported, thus indicating that the direct addition method involves safety problems.
On the other hand, Japanese Laid-Open Patent Publication No. 1386/'79 discloses a process in which, from the viewpoint of safety, a massive masterbatch containing a cross-linking agent at a high concentration is prepared by using a roll mill, Banbury mixer, kneader or the like to incorporate the cross-linking agent into a monoolefin copolymer rubber at a temperature lower than the decomposition temperature of the cross-linking agent. Subsequently, this masterbatch is utilized to prepare partially cross-linked thermoplastic elastomers with the aid of a Banbury mixer, kneader or the like. However, this process poses other problems.
Specifically, the monoolefin copolymer the rubber must be plasticized in order to incorporate the cross-linking agent into the rubber at a temperature lower than the decomposition temperature of the cross-linking agent. On the other hand, the necessity of employing such a temperature as to cause no decomposition of the cross-linking agent makes it imperative to use a monoolefin copolymer rubber having a high propylene content and a low viscosity and a cross-linking agent having a high decomposition temperature, so that restrictions are imposed on both the monoolefin copolymer rubber and the cross-linking agent. Moreover, judging from the method of preparing the masterbatch, it is desirable to use cross-linking agents in powder form. Use of liquid cross-linking agents is disadvantageous from the viewpoint of workability and accuracy of the amount added.
Since the masterbatch thus obtained is in the form of a mass, the preparation of thermoplastic elastomers using it must be carried out not by means of an extruder, but by means of a Banbury mixer or kneader. Thus, in mixing this massive masterbatch with a monoolefin copolymer rubber and a crystalline polypropylene resin, dispersion of the cross-linking agent contained therein requires so much time that the cross-linking agent may begin to decompose before being dispersed to a full extent. As a result, the cross-link density tends to heighten in local areas, leading to the formation of localized tight gel in the resulting thermoplastic elastomer. Since such localized tight gel impairs the appearance and physical properties of products molded from the thermoplastic elastomer, cross-linking agent masterbatches need contain a cross-linking retarder in order prevent the formation of localized tight gel. Nevertheless, where an ethylene-propylene-diene copolymer rubber (EPDM) having a high ethylene content is used as the monoolefin copolymer rubber, it is difficult to prevent the formation of localized tight gel perfectly.
Another method of incorporating a cross-linking agent into a rubber is solution blending. Specifically, the rubber and the cross-linking agent are dissolved in a good solvent, blended and then reprecipitated from the solution. Though feasible in laboratories, this method is unsuitable for industrial purposes because it requires large amounts of organic solvent.
On the other hand, there is a well-known method of cross-linking a granular polyolefin resin by heating the resin in an aqueous medium to impregnate it with a cross-linking agent such as an organic peroxide or the like (Japanese Patent Publication No. 39264/'77, Japanese Laid-Open Patent Publication No. 34436/'72, and Japanese Laid-Open Patent Publication No. 43026/'73). However, the purpose of this method is to cross-link and expand polyolefin resins. Generally, polyolefin resins have a high degree of crystallinity. Accordingly, if a polyolefin resin is impregnated and cross-linked at a temperature lower than its melting point, differences in degree of impregnation and degree of cross-linking arise between the crystalline and the non-crystalline portion of the polyolefin resin, so that it is impossible to obtain a uniformly cross-linked product. For this reason, the impregnation temperature need to be nearly as high as the melting point of the polyolefin resin. As a result, a fusion inhibitor such as basic zinc carbonate, tricalcium phophate or the like must be added in order to prevent the granules of the polyolefin resin from fusing together.